Heretofore, an image projector has been widespread, which comprises an illumination system including a light source, a spatial light modulator modulating intensity of light from the illumination system, and a projection lens imaging the light modulated with the spatial light modulator. There has been an increasing demand for downsizing of the image projector.
Also, as a light source, an LED and an LD (laser diode) have been popular, and a small-size electronic device such as a mobile phone incorporating an image projector has been developed.
Another kind of image projector using an optical scan unit has been proposed. In this optical scan unit, an optical deflector two-dimensionally deflects a light beam from a light source to two-dimensionally scan a projection plane with spots and form a two-dimensional image using afterimage effects. Such an optical scan unit adopts a polygon mirror, a galvano mirror, MEMS devices manufactured by MEMS (Micro Electro Mechanical System) technology, or the like for an optical deflector deflecting a light beam from the light source.
Such an optical scan unit has a problem that projected images may have various kinds of deformations due to the two-dimensional scanning. In view of solving this problem, various attempts have been made.
One example is an optical scan system as an f-arcsine θ lens in a rotationally asymmetric shape to achieve constant speed scanning and correct image deformation, as disclosed in Japanese Unexamined Patent Application Publication No. 2006-178346 (Reference 1) for example.
Another example is an optical scan system having a rotationally asymmetric reflective face to correct trapezoidal deformations or TV distortions, as disclosed in Japanese Unexamined Patent Application Publication No. 2005-234 157 (Reference 2) for example.
Still another example is an optical correction system as a columnar lens with a concave face which corrects unevenness in brightness of an image caused by scan angles, as disclosed in International Patent Application Publication No. WO 2005/083493 (Reference 3) for example.
There is an increasing demand for an image projector mounted in a small-size electronic device as a mobile phone to perform a close range projection in which larger two-dimensional images are formed in a very close range from the device to a projection plane. The close range projection includes not only vertical projection (onto a plane orthogonal to a traveling direction of a scanning light beam) in a close range but also projecting a two-dimensional image 62 on a plane 61 on which a mobile phone 60 is placed, as shown in FIG. 29, for example. The mobile phone 60 incorporates an image projector as an ultracompact projection engine. The image projector is required to be able to project good two-dimensional images on the plane 61 and to be an optical system with relatively simple structure unlike ones in the prior art with a complex structure in view of miniaturization.
However, there is a limitation to the deflection angle θ of the optical deflector included in a MEMS device or the like. For example, at the maximum deflection angle θ of the optical deflector being ±8 degrees, a scan angle of a light beam will be ±16 degrees so that obtainable images are limited to ones with total angle of field of 32 degrees. To form an A4 size projected image using the optical deflector, the small-size electronic device will need a long distance over 50 cm.
Therefore, it is necessary to deflect a light beam at an increased angle with the optical deflector in order to achieve the close range projection in the optical scan unit which two-dimensionally scans.
However, the optical scan systems disclosed in References 1 and 2 are so complex and large structures that they are not suitable for small-size electronic devices as a mobile phone. They are not suitable for achieving size reduction and close range image projection at the same time.
Further, the optical scan system disclosed in Reference 3 corrects a deflection angle of light to make spot scan speed constant on an image plane (projection plane) for the purpose of achieving uniform brightness distribution. However, it fails to disclose or teach a compact scan system which is able to project a large image on a plane in a very close range.